Optical and Supersoft X-ray Light Curve Models of Classical Nova V2491 Cygni: a New Clue to the Secondary Maximum

V2491 Cygni (Nova Cygni 2008 No.2) was detected as a transient supersoft X-ray source with the Swift XRT as early as 40 days after the outburst, suggesting a very massive white dwarf (WD) close to the Chandrasekhar limit. We present a unified model of near infrared, optical, and X-ray light curves for V2491 Cyg, and have estimated, from our best-fit model, the WD mass to be 1.3±0.02 M⊙ with an assumed chemical composition of the envelope, X = 0.20, Y = 0.48, XCNO = 0.20, XNe = 0.10, and Z = 0.02 by mass weight. We strongly recommend detailed composition analysis of the ejecta because some enrichment of the WD matter suggests that the WD mass does not increase like in RS Oph, which is a candidate of Type Ia supernova progenitors. V2491 Cyg shows a peculiar secondary maximum in the optical light curve as well as V1493 Aql and V2362 Cyg. Introducing magnetic activity as an adding energy source to nuclear burning, we propose a physical mechanism of the secondary maxima. Subject headings: novae, cataclysmic variables — stars: individual (V1493 Aql, V2362 Cyg, V2491 Cyg) — stars: mass loss — X-rays: binaries